The present invention relates to measurement techniques of quantum systems operating in the microwave frequency domain, such as superconducting quantum circuits, and more specifically, to detection and/or counting of single microwave photons in a nondemolition manner.
A photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation. A photon carries energy proportional to the radiation frequency and has zero rest mass.
One reason why the detection of single microwave photons is an outstanding challenge is because the energy of a single microwave photon is very small. The energy of a photon in the microwave domain, for example in the range 1-10 gigahertz, is at least 104 times smaller than the energy of a visible light photon.
Circuit quantum electrodynamics (cQED) is one of the leading architectures for realizing a quantum computer based on superconducting microwave circuits. It employs artificial atoms made of nonlinear superconducting devices called qubits which are dispersively coupled to microwave resonators, i.e., the frequencies of the qubits and resonators are detuned. As one example, each superconducting qubit may comprise one or more Josephson junctions shunted by capacitors in parallel with the junctions. The qubits are capacitively coupled to two-dimensional (2D) planar waveguide resonators or three-dimensional (3D) microwave cavities. The electromagnetic energy associated with the qubit is stored in the Josephson junctions and in the capacitive and inductive elements forming the qubit. To date, a major focus has been on improving lifetimes of the qubits in order to allow calculations (i.e., manipulation and readout) to take place before the information is lost due to decoherence of the qubits.
Dispersively coupling a superconducting qubit to a microwave resonator in a cQED architecture loads the resonator and makes its resonance frequency dependent on the quantum state of the qubit (i.e., the resonance frequency of the resonator is different depending on whether the qubit is in the ground or excited states). This property enables the performance of quantum nondemolition measurement of the qubit state, by sending a microwave signal on the order of a few photons to the cQED near the resonator frequency, and measuring the amplitude and/or phase of the output microwave field that carries information about the qubit state. Thus, one potential application of a working and reliable single photon detector in the microwave domain is to enable measuring this weak output signal (i.e., detecting the qubit state) inside the dilution fridge, without requiring the use of high-gain, low-noise, and high-isolation output chains that are typically used nowadays in order to perform such measurements.